TORONTO - Researchers have been given the go-ahead to test stem cells from cord blood with the ultimate goal of reversing hearing loss in infants and toddlers whose inner ears have been damaged.

U.S. regulators have approved the Phase I study, which has a primary objective of determining the safety of the experimental stem cell therapy.

The therapy involves transfusing a baby's own stem cells from umbilical cord blood, banked by parents after their child's birth. Ten children aged six weeks to 18 months old with sensorineural hearing loss will be recruited for the study by doctors at Children's Memorial Hermann Hospital in Houston.

"We're looking more at the ones that suffer an injury around birth or shortly after birth," said Dr. Samer Fakhri, a specialist in head and neck surgery and principal investigator of the study.

Fakhri, a Montreal native who received his medical training at McGill University, said such injuries to the inner ear can be caused by viral infections and even some medications.

Sensorineural hearing impairment occurs when structures in the inner ear or the nerve pathways between the inner ear and the brain are damaged. The critical structure in the inner ear is the snail-shell-shaped cochlea, which contains "hair cells" that gather electrical signals, which are transferred to the brain and perceived as sound.

A child with this kind of hearing loss can suffer significant impairment, Fakhri said from Houston. "You may hear parts of sounds. You may not hear the sounds at all, or you may hear very faint sounds.

"If they lose hearing at four weeks or five weeks due to a viral infection" — meningitis is a common cause — "we know that there is a tremendous impact," he said.

"There's a lot of research that has been done in child development that has determined that there's really a critical window for children to develop speech, language and social development, and it's probably in the first 18 months."

The idea for the trial was triggered by a 2008 study by European scientists, who infused human cord blood into laboratory mice with induced sensorineural hearing loss. An examination of the treated animals about two months later showed "inner ear organization and structure were basically restored," said Fakhri.

"That was the study that was a proof of concept ... That was such a dramatic result."

Fakhri said the exact role of the stem cells in the repair of damaged tissue in the mice isn't known, but there are a couple of theories.

Stem cells can give rise to many different types of cells in the body, so it may be they effect the repair by regenerating lost hair cells. But a more recent theory suggests that stem cells may go to the site of injury and set off the body's innate repair mechanisms.

"In that sense, they play more of a supporting role," he said.

While regenerating tissues is the great hope of stem cells — and they do appear to hold a lot of promise — the idea that they could restore damaged hearing in humans is still speculative, doctors say.

"This study is really very, very preliminary," said Dr. Robert Harrison, a professor of head and neck surgery at the University of Toronto.

"That's the safety issue," he said, stressing that the FDA-approved study must first ensure the stem cells do no harm to patients. Figuring out if they actually work to repair the organ of hearing would have to be proven in subsequent trials.

"We're a long way from looking at the possible therapeutic value of this in terms of restoring some sort of hearing," said Harrison, a senior scientist at the Hospital for Sick Children and a director of the Hearing Foundation of Canada.

"It's a very theoretical concept, and in my opinion it's not going to happen soon."

Current treatment of sensorineural hearing loss in young children is pretty well restricted to hearing aids or cochlear implants, surgically implanted electronic devices, Fakhri said. Both are used to amplify any residual hearing.